Pipelines are used to transport a variety of fluids. For example, pipelines often transport oil, gaseous hydrocarbons, water, chemical feedstocks, etc. However, under some circumstances, the presence in a pipeline of a particular type of fluid may be undesirable or detrimental either to the fluid intended to be transported or to the pipeline itself. For example, the presence of oxygen in a subsea pipeline for transporting liquid petroleum is undesirable because it promotes corrosion of the pipeline. As further example, the presence of water in a pipeline for transporting natural gas is undesirable and the presence of oil in a water pipeline can be undesirable. As used herein, “pipeline” includes reference to any particular section or portion of a pipeline.
Government regulations may require that pipelines for transporting hazardous liquids, natural gas, or petroleum products, be tested before being put into service to ensure there are no leaks in the pipeline. Even in the absence of government requirements, pipelines are commonly tested for leaks before being put into service. Pipelines may also be tested for leaks after repairs are performed. Hydraulic pressure testing is commonly performed to test a pipeline for leaks. Typically, hydraulic pressure testing is performed by pressurizing a pipeline with water to a specific pressure for a period of time. Loss of pressure would be evidence of a leak in the pipeline. After such hydraulic testing, however, it may be necessary to clear the pipeline of water before transporting the desired product.
A “pig” can be used to clear a pipeline of an undesired fluid. A pig is typically deployed in one section of the pipeline (referred to herein as the “deployment” of a pig), propelled through the pipeline, and recovered at another section of the pipeline (referred to herein as the “recovery” of a pig). Use of a pig to remove undesirable fluid from a pipeline is referred to herein as “sweeping” and each individual journey of a pig from deployment to recovery is referred to herein as a “sweep.” As used herein, a “downstream” direction is the direction of a pig's travel along a pipeline during a sweep and an upstream direction is the direction along a pipeline opposite to the direction of a pig's travel along a pipeline during a sweep. Typically, a fluid is used to pressurize the section of pipe upstream of a pig propelling the pig downstream.
A “sweeping pig” generally contacts the inner surface of a pipeline and preferably maintains such contact with the inner surface during a sweep. During a sweep of a pipeline, a sweeping pig pushes contents of a pipeline downstream thereby removing such contents from the portion of the pipeline traversed by the pig. In practice, however, contact between the pig and the inner surface of the pipeline is imperfect and some of an undesirable fluid may remain in the pipeline. Several sweeps with a pig are typically required to ensure that an undesirable fluid is removed from the pipeline. The number of sweeps required can be many dozen if, for example, the pipeline has a complex profile with numerous turns, changes in elevation or extended inclines.
Sweeping pigs are typically cleaning pigs used in the art for cleaning the inside of a pipeline. Many types of cleaning pigs exist including pigs comprising foam, pigs with brushes or other scrubbing devices, or many others known and used in the art.
Sweeping pigs for sweeping pipelines such as described above can become stuck in a pipeline during a sweep. Considerable effort and time are required to extract a stuck sweeping pig from a pipeline and may result in damage to either the pig or the pipeline or both. A stuck pig usually prevents operation of a pipeline until it the pig is extracted. The more sweeps that are conducted with a sweeping pig, the greater the probability of the pig becoming stuck or otherwise damaged, and the greater probability of delay and damage to the pipeline or other equipment. It is therefore preferable to conduct no more sweeps than are necessary to remove an undesirable fluid from a pipeline. Extra sweeps are typically needed to ensure that an undesirable fluid is removed from a pipeline, but such extra sweeps increase the time, effort, and expense of a sweeping operation and increase the probability of a pig becoming stuck. Extra sweeps could be advantageously eliminated if the presence or absence of an undesirable fluid within a pipeline could be detected.
For example, an existing pipeline for the transportation of a fluid comprising subsurface hydrocarbon gases was swept to remove water after hydrostatic testing. The 40 mile stretch of pipeline was swept using prior art apparatus and equipment. 60 sweeps were performed and there was still no certainty regarding whether water was present in the pipeline. During the operation, 2 pigs became stuck in a check valve requiring a 3 day repair operation.
Other pigging devices exist for detecting irregularities, corrosion or other physical abnormalities in a pipeline. For example, U.K. Patent number 2,305,989 discloses a pigging apparatus for obtaining a substantially continuous pipeline condition profile and storing data to record a pipeline condition profile. Pigs with such monitoring and data storage apparatus are more expensive and have greater sensitivity to the stresses of sweeping a pipeline than traditional sweeping pigs. Typically, such pigs with monitoring and data storage devices are employed after undesirable fluid is removed because such fluid may be detrimental to the functioning of the monitoring and data storage apparatus. Additionally, in an operation requiring multiple sweeps, such as is typically the case when sweeping a pipeline of undesirable fluids, multiple attempts at detecting the presence of an undesirable fluid may be needed. Each additional detection attempt increases risk of damage to the pig, the pipeline and other equipment.
There is a need in the art for a simple method and apparatus for detecting the presence or absence of particular fluids in a pipeline. There is also a need for a method and apparatus to reduce the number of sweeps conducted when sweeping a pipeline to remove an undesirable fluid.